1. Field of the Invention
This invention relates to the inhibition of oxidation in lubricants. More particularly, the invention has to do with lubricants in which have been placed an additive having the ability to reduce oxidation of such lubricant. Still more particularly, the additive of this invention may be referred to as an ester of an N-naphthyl anthranilic acid.
2. Discussion of the Prior Art
Lubricants, such as lubricating oils and greases, are subject to oxidative deterioration at elevated temperatures or upon prolonged exposure to the elements. Such deterioration is evidenced, in many instances, by an increase in acidity and in viscosity, and when the deterioration is severe enough, it can cause metal parts to corrode. Additionally, severe oxidation leads to a loss of lubrication properties of the lubricants, and, in especially severe cases, to complete breakdown of the device being lubricated. In combatting oxidation, many additives have been tried, but many of them are only marginally effective except at high concentrations, especially when the lubricant is subjected to drastic oxidizing conditions.
There are patents which disclose certain anthranilic acid esters as antioxidants. U.S. Pat. Nos. 2,369,090 and 2,390,943 disclose methylanthranilate for this purpose in a petroleum or hydrocarbon oil. U.S. Pat. No. 2,369,090 also teaches the use of ethyl, propyl or butyl anthranilate. U.S. Pat. No. 3,642,632 is concerned with the C.sub.1 -C.sub.10 alkyl anthranilates and those N-substituted anthranilates where the substituent is a C.sub.1 -C.sub.6 alkyl or phenyl. Such anthranilates are said to be effective antioxidants, especially for synthetic ester oils. It will be shown that the N-naphthyl anthranilates used in the practice of this invention are unexpectedly superior to those disclosed in the patents mentioned.